This invention relates to the field of inkjet printing, more particularly to the correction of image artifacts produced by errors in the placement of ink drops printed on a receiver and to methods of guiding ink drops to receivers to produce prints of high image quality.
As is well known in the art of inkjet printing, the quality of printed images suffers from the misplacement of a portion of the printed ink drops from their desired print location. Such a misplacement of ink drops may repeatedly occur for all drops ejected by a particular nozzle, because the drops are ejected at an angle different from the desired angle of ejection (i.e., misdirection), for example, as a result of a fabrication defect in the respective nozzle. Alternatively, misdirection may randomly occur from time to time for drops ejected from one or more nozzles, due to physical changes in the nozzle or the environment of the nozzles; for example, changes caused by prolonged heating of a particular nozzle from extended use of that nozzle, or from passage of certain particulates through the nozzle. Also, difficult-to-control interactions between the ink, impurities in the ink, and the nozzle surfaces constitute a random variation that is well known in the art. The forces of nozzle surface tension can cause random misdirection of ejected drops. Random variations in the angle of drop ejection may also occur due to uncontrolled air currents in the vicinity of the nozzles.
Repetitive or consistent variations in the angle of drop ejection of a particular nozzle may be controlled by measuring the degree of variation and correcting for it, using one or more means of correction for drop placement, as disclosed, for example, in co-pending U.S. patent application Ser. No. 09/586,099, filed Jun. 2, 2000, by Hawkins et al., and entitled, xe2x80x9cPermanent Alteration Of A Printhead For Correction Of Mis-Direction Of Emitted Ink Drops,xe2x80x9d which discloses methods for permanently altering the geometry of nozzles, and references therein. However, random variations are more difficult to control, because the angle of drop ejection changes over the life of the printhead and the aforementioned correction means cannot be applied. Such print compensation, while possible, requires a costly measurement apparatus to determine whether all ink drops pass through all predetermined orifices and at least some drops are not printed in their desired print locations, since misdirected drops must be observed in order to have their direction of ejection corrected.
Another strategy for correcting slowly changing variations in the direction of drop ejection is disclosed in U.S. Pat. No. 4,238,804, by Warren, Dec. 9, 1980, assigned to Xerox Corporation, and U.S. Pat. No. 3,877,036, by Loeffler et al., Apr. 8, 1975, assigned to IBM, which teach measuring the position of ejected ink drops and compensating for variations from the ideal direction by electrostatic means. While such electrostatic deflection can be used to direct ink in a desired direction, as is well known in the art, electrostatic deflection in these cases adds mechanical complexity. Also, correction techniques of this type are largely ineffective in cases where large variations in the direction of ejected ink drops occur.
U.S. Pat. No. 5,592,202, by Erickson, Jan. 7, 1997, assigned to Laser Master Corporation, teaches an electronic means to correct inaccuracies in ink drop placement by advancing or retarding the time of a drop-on-demand actuation pulse. However, this method does not correct variations in both of the directions of ink drop ejection in a plane perpendicular to the direction of drop ejection, as it is more suited to adjusting ink drop placement only in the scan direction of the printhead. Moreover, not all printhead circuits can be easily adapted to control the firing times of individual ink drops, since the firing pulses may be derived from a common clock. Also, at least some drops are printed in locations other than their desired print locations, since drop misplacement must be observed in order to be corrected.
U.S. Pat. No. 5,250,962, by Fisher et al., Oct. 5, 1993, assigned to Xerox Corporation, teaches the removal of entrained air in one or more nozzles to correct for drop misdirection without the necessity of measuring the degree of misdirection. However, although entrained air is known in the art to cause variations in the direction of ink drop ejection, it is only one of many mechanisms causing variations.
U.S. Pat. No. 4,914,522, by Duffield, et al., Apr. 3, 1990, assigned to Vutek Inc., discloses a drop-on-demand ink jet printer that utilizes air pressure to produce a desired color density in a printed image. Ink in a reservoir travels through a conduit and forms a meniscus at an end of an inkjet nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the ink nozzle, causes the ink to be extracted from the nozzle and atomized into a fine spray which lands on a receiver. The stream of air is applied at a constant pressure through a conduit to a control valve opened and closed by a piezoelectric actuator. When a voltage is applied to the valve, the valve opens to permit air to flow through the air nozzle. When the voltage is removed, the valve closes and no air flows through the air nozzle. While the desired density of the ink on the receiver can be varied on average within a printed pixel region by varying the pulse width of the airstream, the drops so produced arise from many places on the meniscus, are of many sizes, are ejected at many different angles, and land in a variety of places on the receiver, even when only a single pixel is printed, due to the turbulence of the airstream and its role in pulling drops off the meniscus, as can be appreciated by one skilled in the art of air-meniscus interactions. No two single pixels would be printed identically when the precise position of the drops is considered. Additionally, the airstream must be turned on and off repeatedly so that a steady, equilibrium airflow is never attained.
Other techniques for achieving compensation include the selection of one nozzle among a plurality of redundant nozzles for printing a particular imaging pixel, the preferred nozzle having favorable ink drop ejection characteristics. However, redundancy selection techniques of this type are complex in nature and require substantial real estate space on the printhead. Such methods also increase cost and/or reduce productivity, and again, at least some drops may not printed in their desired print locations, since a failed nozzle must be observed in order to be replaced by a redundant nozzle.
U.S. Pat. No. 5,815,178, by Silverbrook, Sep. 29, 1998, describes a means for partially correcting drop placement errors that does not require observing or printing misdirected drops and thus is cabable of correcting truly random variations in the direction of drop ejection. According to this method, the use of high electric fields to pull the drops toward a direction of field lines perpendicular to the plane of the nozzle""s surfaces, thereby helping guide all drops ejected from all nozzles toward their respective desired print locations. Since all drops are guided toward their respective desired print locations, whether they are misdirected or not, the electric field automatically corrects drop placement errors resulting form all types of drop misdirection, random or constant. However, the electric field of Silverbrook, to effectively accomplish its purpose, must be very large and consequently produces undesired electrical arcing.
Thus, it is desirable to provide a device and method of operation of an inkjet printhead that provides correction for ink drop placement errors, including random misdirection of the angles at which ink drops are ejected, accordingly being advantageous to print quality without penalty of print productivity and cost and which is capable of repeatedly and predictably placing drops in exact locations desired for printing without perturbing the drop ejection process.
The present invention provides a device and a method of operation of an inkjet printhead, that corrects for drop placement errors, including random misdirection of the angles at which drops are ejected. Such a method is advantageously accomplished without the need to measure the direction of ejection of drops.
One feature of the present invention is that the trajectories of drops that are initially ejected in a direction other than that of a desired direction are continuously corrected over a substantial portion of their time of flight from the nozzle to the receiver.
Another advantageous feature of the present invention is that the device and method do not require energy consuming means to redirect misplaced drops.
It is yet another advantage of the present invention that the device and method may be applied advantageously to a variety of types of drop ejectors, including continuous and drop-on-demand ejectors.
Still another advantage of the present invention is that the distance from the nozzle to the receiver may be made larger than would otherwise be possible.
It is a further advantage of the present invention that the cost of the present invention does not substantially increase with the number of printhead nozzles.
The present invention is directed to overcoming one or more of the problems set forth above by providing an apparatus for controlling errant ink drops in an inkjet printer having a plurality of nozzles for ejecting ink drops along a droplet trajectory and printing the ejected ink drops onto a receiver, including: a) at least one airflow channel arranged to provide a non-uniform airflow pattern located along a portion of the droplet trajectory, wherein the apparatus is in close proximity to the plurality of nozzles and prior to the receiver, such that the non-uniform airflow pattern provides compensation for errors in the printing of the ejected ink drops on the receiver, and b) means for moving air in the airflow channel; and by providing a method of printing ink drops onto a receiver to desired printing locations, comprising the steps of: a) providing an airflow guide channel to guide the printed ink drops, b) ejecting ink drops from a printer nozzle, c) directing a non-uniform airstream through the airflow channel to cause errant ink drops to automatically correct before placement on the receiver regardless of any initial misdirection of the ink drops, and d) printing corrected ink drops onto the receiver.